1. Field of the Invention
The present invention relates to a beam splitter operable in the visible spectrum which reflects one linear polarization and transmits the other. More particularly, the present invention relates to such a beam splitter which utilizes a wire grid polarizer as the operative optical element. In addition, the present invention relates to the key parameters of the beam splitter to obtain desirable function throughout the visible spectrum.
2. Prior Art
Wire grid polarizers (WGPs) have been used in polarizing optical systems but have not been effectively applied in beam splitters. For example, wire grid polarizers have been developed which operate in the infrared and longer wavelengths..sup.1 Structures with grid spacings or periods as fine as 0.115 .mu.m have been reported..sup.2 Many concepts which enhance the performance of wire grid polarizers have been taught. For example, Garvin.sup.3 and Keilmann.sup.3 teach ways of improving the performance of wire grid polarizers operating in the infrared at normal incidence. As another example, Tamada teaches the concept of using resonance effects in grating structures to produce a narrow spectral band polarizing element that does not require that the grid spacing be much less than the wavelength of the incident light..sup.5 But a wire grid polarizer that operates over a broad spectral band, e.g. the visible spectrum, requires that the grid spacing be much less than the wavelength of the incident light. One disadvantage with Tamada is that he does not teach a polarizing beam splitter which operates at a given angle and with a given ratio of intensity between the split beams. Tamada, like others, only discusses structures operating near normal incidence.
The concept of using infrared wire grid polarizers at large angles of incidence is occasionally discussed in the literature. For example, Stenkamp measured the transmission of a wire grid polarizer with a period of 100 nm at an angle of incidence of 80.degree.. Stenkamp observed an increase in the extinction ratio at a wavelength of 670 nm. Stenkamp, however, did not measure the reflected radiation intensity, and the data are for only one wavelength..sup.6 As another example, Bird stated that qualitative tests of the effect of oblique incidence showed that the transmittance of the wire grid was nearly independent of oblique incidence up to 30.degree. off-normal,.sup.7 in agreement with a careful study by Pursley..sup.8
The Handbook of Optics states that wire grid polarizers can be used in optical systems with high numerical apertures..sup.9 Specifically, Young is cited as finding no decrease in the percent of polarization for a mid-IR (12 .mu.m) polarizer used at angles of incidence from 0.degree. to 45.degree. while transmittance decreased by more than 30% (0.55 to less than 0.40)..sup.10
Key grid parameters that can be used to design polarizing beam splitters include period (p), line width (w), line depth or thickness (t), properties (e.g., index of refraction) of the grating material, properties of the substrate material (e.g., index of refraction), angle of incidence, the wavelength of the incident radiation and grating resonance effects, e.g. the Rayleigh resonance. For example, Haggans studied the effect of these parameters on optical beams reflected from the wire grid..sup.11 It appears that most of Haggans calculations are for a 45.degree. angle of incidence and transmission is not considered. As another example, Schnable states that changing the metal material is not very useful since there are only a few exceptions where one can increase the polarization effect compared with chromium for a certain wavelength range..sup.12
In addition, Haidner describes a polarizing reflection grating polarizer that works only at normal incidence and one wavelength (10.6 .mu.m)..sup.13
It is desirable to have a wire grid polarizing beam splitter with a high and uniform transmission efficiency across the visible spectrum; a high and uniform reflection efficiency across the visible spectrum; a high transmission and/or reflection extinction across the visible spectrum, a large numerical aperture, e.g., transmission and reflection efficiencies and extinctions must be maintained across an appreciable light cone; and work well with a light cone whose cone angle is as large as 20 to 30.degree.. In order to meet these criteria, a practical design that has uniform performance across the entire visible spectrum must account for and control grating resonance effects such as the Rayleigh resonance. As indicated above, some references discuss some aspects of the grid parameters that affect performance of wire grid polarizers at oblique angles, while other references reveal confusion about the role of these same parameters. None of the references, however, bring together the key concepts necessary to the design of a useful wire grid polarizing beam splitter for imaging applications in the visible spectrum.
The key concepts or physical parameters that must be interrelated and addressed collectively to ensure the desired degree of functionality for a wire grid polarizing beam splitter include: the structure and shape of the grid profile; the height or thickness of the wires or grid elements; orientation of the grid with respect to the polarizations of light; the grid materials; incidence angle; convergence, divergence or cone angle; and the effects of phenomena such as Rayleigh resonance. These concepts must be understood in order to obtain the desired functionality of an effective wire grid polarizing beam splitter.
Therefore, it would be advantageous to develop a beam splitter using a wire grid polarizer for efficiently reflecting one linear polarization and transmitting the other over a broad spectral range. It would also be advantageous to develop such a beam splitter capable of being positioned at a variety of incidence angles so that significant design constraints are not imposed on the optical system, but substantial design flexibility is permitted. It would also be advantageous to develop such a beam splitter which accounts for various important design concepts or parameters, such as wire grid profile, wire grid dimensions, wire grid material, wire grid orientation, wavelength range, incidence angle, cone angle, Rayleigh resonance effects, etc. It would also be advantageous to develop such a beam splitter with a large acceptance angle capable of accepting relatively divergent light.
1. H. Hertz, Electric Waves (Macmillan and Company, Ltd., London, 1893) p.177.; G. R. Bird and M. Parrish, Jr., "The Wire Grid as a Near-Infrared Polarizer," J. Opt. Soc. Am. 50, pp.886-891, 1960. PA0 2. G. J. Sonek, D. K. Wanger, and J. M. Ballantyne, Appl. Opt. 22, pp. 1270-1272, 1983. PA0 3. Garvin, U.S. Pat. No. 4,289,381 PA0 4. Keilmann, U.S. Pat. No. 5,177,635 PA0 5. Tamada, U.S. Pat. No. 5,748,368; and H. Tamada, et al., "Al wire-grid polarizer using the s-polarization resonance effect at the 0.8-.mu.m-wavelength band," Optics Letters, 22, No. 6, pp. 410-421, 1996) PA0 6. B. Stenkamp, et al., "Grid polarizer for the visible spectral region," SPIE, 2213, pp. 288-296 (1994) PA0 7. G. R. Bird and M. Parrish, Jr., "The Wire Grid as a Near-Infrared Polarizer," J. Opt. Soc. Am., 50, pp. 886-891 (1960) PA0 8. W. K. Pursley, Doctoral thesis, University of Michigan, (1956). PA0 9. Michael Bass, Editor in Chief, The Handbook of Optics, Volume II, p. 3.34, McGraw-Hill, Inc., New York (1995) PA0 10. J. B. Young, et al., Appl. Opt. 4, pp. 1023-1026 (1965) PA0 11. C. W. Haggans, et al., "Lamellar gratings as polarization components for specularly reflected beams," J. Mod. Optics, 40, pp. 675-686 (1993) PA0 12. B. Schnable, et al. "Study on polarizing visible light by subwavelength-period metal-stripe gratings" Opt. Eng. 38(2), pp. 220-226 (1999) PA0 13. H. Haidner, et al., "Polarizing reflection grating beamsplitter for 10.6-.mu.m wavelength," Opt. Eng., 32(8), 1860-1865 (1993)